In general, the systems used to control the friction brakes on vehicles, be they railway trains, aircraft or automobiles, carry out braking commands by applying to the friction elements forces sized to the command. Selection of the relationship between the command an the resulting braking force involves assumptions regarding those factors, such as the coefficient of friction of the braking elements and the adhesion of the wheels, which affect vehicle motion. Therefore, the braking force levels are chosen on an assumed average performance. As a result, under any given set of conditions, some wheels may be braked too much and others may be braked too little. Because of this, only approximate correspondence between commanded acceleration and actual acceleration is afforded.
The adverse effects of the assumptions involved in the design of the traditional brake system are manifest in various forms, e.g. excessive stopping distances, excessive wheel wear resulting from slip, and the creation of flat spots on the wheels or the complete loss of steering control resulting from locked or sliding wheels. In an effort to improve performance, various refinements has been proposed. One involves weight sensing apparatus which reduces the braking forces on a localized basis as the load borne by the wheel, and consequently its adhesion, is reduced. Another type of refinement is the wheel slip control which releases and then reapplies individual brakes when slipping of the associated wheel is detected. A third remedial measure consists of a torque feedback system which is intended to linearize the relationship between the commanded force (or pressure) applied to the friction element and the friction force developed between this element and its cooperating braking part. While these expedients do improve performance, all are complex and increase substantially the cost of manufacturing and maintaining the system. Moreover, and more important, this auxiliary apparatus performs an override of reaction role and thus is incapable of deriving from each wheel all of the braking effort which its localized adhesion will support. Therefore, even with the auxiliary equipment, the system still affords stopping distances much longer than the theoretical minimum.
One object of this invention is to provide an improved braking scheme which affords stopping distances much shorter than those heretofore attainable, precludes wheel sliding and minimizes slipping, and does not require auxiliary devices of the type mentioned above. According to this invention the braking command controls the tangential velocities of the wheel treads, and the forces applied to the braking elements are controlled indirectly on an individual basis and as required to maintain equal the tangential speeds of all wheels. With this scheme, all of the wheels are compelled to accelerate at the same identical rate determined exclusively by the command; consequently, all of the variables, such as vehicle weight, coefficient of friction of the braking elements, and localized adhesion values, which influence motion of the vehicle are inherently and completed compensated. As a result, the actual acceleration of the vehicle is rendered independent of these conditions, and no apparatus need be provided to detect and correct specifially for them. In situations where local track or road conditions at a wheel are such that the commanded rate or retardation or acceleration cannot be supported, the wheel will simply cease to contribute to the total braking or accelerating effort, and its share of this effort will be apportioned among the remaining wheels. In no event will such a wheel slide, i.e., lock-up, and it can slip, i.e., rotate at a tangential speed less than vehicle speed, only in cases where all, or most, of the wheels on the vehicle are in a similar adhesionless state. This would be an extremely rare occurrence in the case of railway service, but it could happen in the case of an automobile or an aircraft. However, even in those applications, steering capability would be considerably better than under the sliding condition which the conventional system would create, and each wheel will be available immediately to again contribute braking effort as soon as it encounter ground conditions which provide the adhesion needed to support the commanded acceleration.
The basic principle of the invention is independent of the sense of vehicle acceleration, and therefore may be employed in the propulsion as well as the braking mode of of operation. This application of the new scheme is of particular importance in the case of railway rapid transit service, where all the axles of the individual cars are equipped with traction motors, because it affords an effective way of furnishing higher acceleration rates without jerks or wheel spin than heretofore possible. Furthermore, when these motors are used as dynamic brakes, the invention inherently modulates the friction brake as needed to offset changes in the effectiveness of the dynamic brake, and thus obviates inclusion of auxiliary brake-blending devices.
Regardless of whether the acceleration-imparting mechanisms associated with the wheels are friction brakes or propulsion motors or both, the preferred form of the invention uses a master command signal in the form of a variable frequency electrical signal. This signal is transduced by synchronous motors to provide, for each acceleration-imparting mechanism, rotary motion of an input shaft at a velocity proportional to the frequency. This input motion is compared to the motion of a feedback shaft which is driven at an angular velocity proportional to the tangential speed of the tread of the associated wheel, and an actuating mechanism adjusts the acceleration-imparting mechanism as required to maintain synchronized each pair of input and feedback shafts. The angular displacement of each input shaft is proportional to the cumulative cycles of the master signal, and the corresponding displacement of the feedback shaft is proportional to the linear travel of the wheel. Therefore, since the shafts are maintained synchronized within a small control range, it follows that this version of the invention not only affords the desired speed control, but also allows the distance traveled by the vehicle to be regulated by merely controlled the total number of cycles of the master signal. Obviously, this feature makes the invention particularly attractive for use in automatic train control systems.